1. Field of the Invention
The invention relates to a monitor pattern for photolithography, and in particular to a monitor pattern used for concurrently detecting resolutions thereof and determining whether the resolutions are affected by leveling.
2. Description of the Prior Art
As we know, photolithography is a very important step in the entire semiconductor processing. Basically, any regions related to the structure of semiconductor devices, such as subsequently pattered and doped regions, are all previously defined by photolithography. Thus, the complexity of the semiconductor processing depends on the times of photolithography performed and the number of masks used.
To perform the photolithography, a photo-sensitive material is formed on a wafer. A parallel light coming from a light source passes through a glass-based mask and irradiates the photo-sensitive material. At the same time, a pattern on the mask is projected onto the photo-sensitive material. Next, development is performed to accomplish a pattern transfer.
In general, an aligner is used to perform photolithography based on alignment marks on wafers. However, an optical system in the aligner has a problem of astigmatism which creates an aberration, resulting in a poor pattern transfer.
FIG. 1 is a schematic view illustrating an aberration caused by a problem of astigmatism on the optical system of an aligner. As shown in FIG. 1, a first image point 110 and a second image point 112 of a pattern point 106 are separately projected onto an optical axis 108 by a lens 100 due to the difference existing between a meridional focal plane 102 and a horizontal focal plane 104. Therefore, an aberration AST, which is equal to the distance between the two image points 110 and 112, is created.
Furthermore, projected image lines projected by the lens 100 must accurately maintain at a width of more than a critical dimension (CD) so as to allow the image lines to be located within a range of effective resolution.
However, if the aberration AST is increased, it causes that images projected on the meridional focal plane 102 and horizontal focal plane 104 have different resolutions, decreasing an effective depth of forces (DOF) and differing the critical dimensions of the images. So far, there are still no simple standard specifications to judge the aberration for photolithography. Frequently, a lot of complicated testing processes are required to quantitatively analyze the aberration. As a result, the photolithography cannot be efficiently monitored.
FIG. 2 is a schematic view illustrating how the thickness of a photoresist layer affects the pattern transfer in photolithography. As shown in FIG. 2, a pattern on a mask (not shown) is transferred onto a photoresist layer 122 formed on a wafer 120 by photolithography. Since the photoresist layer 122 has a certain thickness d (generally 1 xcexcm), a certain depth of focus (DOF) is required to completely and precisely project the pattern onto the photoresist layer 122 by a light wave 124. Thus, the projected pattern on the photoresist 122 can have a better resolution regardless of on the top surface 126 of the bottom surface 128 thereof.
FIG. 3 is a schematic view showing a problem of leveling when an optical system performs an image projection. As shown in FIG. 3, the surface 300 of a wafer has a relief and curve feature where part 301 thereof is obviously out of the range of the depth of focus (DOF) (from a best focus BF to both alignment focuses AF). As a result, a required pattern projected onto a photoresist layer (not shown) formed on the wafer has a poor resolution. To overcome this problem, it is necessary to take more time and man power to frequently adjust the leveling of a used aligner thereby to obtain a precise transferred pattern by a complicated testing process.
In view of the above, an object of the invention is to provide a monitor pattern for photolithography. The monitor pattern of the invention includes to concave quadrangular sub-pattern and a scale sub-pattern. The concave quadrangular sub-pattern has a pair of equal exterior sides, a pair of equal interior sides and a right angle include between the pair of equal exterior sides and is bisected into a horizontal region and a vertical region congruent with each other along a bisector of the right angle. The scale sub-pattern is located along the pair of equal exterior sides for measuring the resolution of the concave quadrangular sub-pattern. In addition, a monitor pattern according to the invention can includes two symmetric concave quadrangular sub-patterns and two symmetric scale sub-patterns for the purpose of further determining whether or not resolutions are affected by leveling.